Implants and methods for treating bone

ABSTRACT

An orthopedic implant comprising a deformable, expandable implant body configured for treating abnormalities in bones, such as compression fractures of vertebra, necrosis of femurs and the like. An exemplary implant body comprises a small cross-section threaded element that is introduced into a bone region and thereafter is expanded into a larger cross-section, monolithic assembly to provide a bone support. In one embodiment, the implant body is at least partly fabricated of a magnesium alloy that is biodegradable to allow for later tissue ingrowth.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of Provisional U.S. Patent ApplicationSer. No. 60/577,559 filed Jun. 7, 2004 (Docket No. S-7700-040) titledReticulated Implants and Methods for Treating Abnormalities in Bone, theentire contents of which are hereby incorporated by reference in theirentirety and should be considered a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to medical implants and more particularly toexpandable open cell reticulated implants configured as bone supportimplants for treating abnormalities in bones such as compressionfractures of vertebra, necrosis of femurs and the like. An exemplaryimplant body comprises a small cross-section threaded element that isintroduced into a bone region and thereafter is expanded into a largercross-section, open cell reticulated monolith to provide a bone support.

2. Description of the Related Art

Osteoporotic fractures are prevalent in the elderly, with an annualestimate of 1.5 million fractures in the United States alone. Theseinclude 750,000 vertebral compression fractures (VCFs) and 250,000 hipfractures. The annual cost of osteoporotic fractures in the UnitedStates has been estimated at $13.8 billion. The prevalence of VCFs inwomen age 50 and older has been estimated at 26%. The prevalenceincreases with age, reaching 40% among 80-year-old women. Medicaladvances aimed at slowing or arresting bone loss from aging have notprovided solutions to this problem. Further, the affected populationwill grow steadily as life expectancy increases. Osteoporosis affectsthe entire skeleton but most commonly causes fractures in the spine andhip. Spinal or vertebral fractures also have serious consequences, withpatients suffering from loss of height, deformity and persistent painwhich can significantly impair mobility and quality of life. Fracturepain usually lasts 4 to 6 weeks, with intense pain at the fracture site.Chronic pain often occurs when one level is greatly collapsed ormultiple levels are collapsed.

Postmenopausal women are predisposed to fractures, such as in thevertebrae, due to a decrease in bone mineral density that accompaniespostmenopausal osteoporosis. Osteoporosis is a pathologic state thatliterally means “porous bones”. Skeletal bones are made up of a thickcortical shell and a strong inner meshwork, or cancellous bone, ofcollagen, calcium salts and other minerals. Cancellous bone is similarto a honeycomb, with blood vessels and bone marrow in the spaces.Osteoporosis describes a condition of decreased bone mass that leads tofragile bones which are at an increased risk for fractures. In anosteoporosic bone, the sponge-like cancellous bone has pores or voidsthat increase in dimension, making the bone very fragile. In young,healthy bone tissue, bone breakdown occurs continually as the result ofosteoclast activity, but the breakdown is balanced by new bone formationby osteoblasts. In an elderly patient, bone resorption can surpass boneformation thus resulting in deterioration of bone density. Osteoporosisoccurs largely without symptoms until a fracture occurs.

Vertebroplasty and kyphoplasty are recently developed techniques fortreating vertebral compression fractures. Percutaneous vertebroplastywas first reported by a French group in 1987 for the treatment ofpainful hemangiomas. In the 1990's, percutaneous vertebroplasty wasextended to indications including osteoporotic vertebral compressionfractures, traumatic compression fractures, and painful vertebralmetastasis. In one percutaneous vertebroplasty technique, bone cementsuch as PMMA (polymethylmethacrylate) is percutaneously injected into afractured vertebral body via a trocar and cannula system. The targetedvertebrae are identified under fluoroscopy. A needle is introduced intothe vertebral body under fluoroscopic control to allow directvisualization. A transpedicular (through the pedicle of the vertebrae)approach is typically bilateral but can be done unilaterally. Thebilateral transpedicular approach is typically used because inadequatePMMA infill is achieved with a unilateral approach.

In a bilateral approach, approximately 1 to 4 ml of PMMA are injected oneach side of the vertebra. Since the PMMA needs to be forced intocancellous bone, the technique requires high pressures and fairly lowviscosity cement. Since the cortical bone of the targeted vertebra mayhave a recent fracture, there is the potential of PMMA leakage. The PMMAcement contains radiopaque materials so that when injected under livefluoroscopy, cement localization and leakage can be observed. Thevisualization of PMMA injection and extravasion are critical to thetechnique—and the physician terminates PMMA injection when leakage isevident. The cement is injected using small syringe-like injectors toallow the physician to manually control the injection pressures.

Kyphoplasty is a modification of percutaneous vertebroplasty.Kyphoplasty involves a preliminary step that comprises the percutaneousplacement of an inflatable balloon tamp in the vertebral body. Inflationof the balloon creates a cavity in the bone prior to cement injection.Further, the proponents of percutaneous kyphoplasty have suggested thathigh pressure balloon-tamp inflation can at least partially restorevertebral body height. In kyphoplasty, it has been proposed that PMMAcan be injected at lower pressures into the collapsed vertebra since acavity exists to receive the cement—which is not the case inconventional vertebroplasty.

The principal indications for any form of vertebroplasty areosteoporotic vertebral collapse with debilitating pain. Radiography andcomputed tomography must be performed in the days preceding treatment todetermine the extent of vertebral collapse, the presence of epidural orforaminal stenosis caused by bone fragment retropulsion, the presence ofcortical destruction or fracture and the visibility and degree ofinvolvement of the pedicles. Leakage of PMMA during vertebroplasty canresult in very serious complications including compression of adjacentstructures that necessitate emergency decompressive surgery.

Leakage or extravasion of PMMA is a critical issue and can be dividedinto paravertebral leakage, venous infiltration, epidural leakage andintradiscal leakage. The exothermic reaction of PMMA carries potentialcatastrophic consequences if thermal damage were to extend to the duralsac, cord, and nerve roots. Surgical evacuation of leaked cement in thespinal canal has been reported. It has been found that leakage of PMMAis related to various clinical factors such as the vertebral compressionpattern, and the extent of the cortical fracture, bone mineral density,the interval from injury to operation, the amount of PMMA injected andthe location of the injector tip. In one recent study, close to 50% ofvertebroplasty cases resulted in leakage of PMMA from the vertebralbodies. See Hyun-Woo Do et al, “The Analysis of PolymethylmethacrylateLeakage after Vertebroplasty for Vertebral Body Compression Fractures”,Jour. of Korean Neurosurg. Soc. Vol. 35, No. 5 (May 2004) pp. 478-82,(http://www.jkns.or.kr/htm/abstract.asp?no=0042004086).

Another recent study was directed to the incidence of new VCFs adjacentto the vertebral bodies that were initially treated. Vertebroplastypatients often return with new pain caused by a new vertebral bodyfracture. Leakage of cement into an adjacent disc space duringvertebroplasty increases the risk of a new fracture of adjacentvertebral bodies. See Am. J. Neuroradiol. 2004 February; 25(2): 175-80.The study found that 58% of vertebral bodies adjacent to a disc withcement leakage fractured during the follow-up period compared with 12%of vertebral bodies adjacent to a disc without cement leakage.

Another life-threatening complication of vertebroplasty is pulmonaryembolism. See Bernhard, J. et al., “Asymptomatic diffuse pulmonaryembolism caused by acrylic cement: an unusual complication ofpercutaneous vertebroplasty”, Ann. Rheum. Dis. 2003; 62:85-86. Thevapors from PMMA preparation and injection are also cause for concern.See Kirby, B., et al., “Acute bronchospasm due to exposure topolymethylmethacrylate vapors during percutaneous vertebroplasty”, Am.J. Roentgenol. 2003; 180:543-544.

Another disadvantage of PMMA is its inability to undergo remodeling—andthe inability to use the PMMA to deliver osteoinductive agents, growthfactors, chemotherapeutic agents and the like. Yet another disadvantageof PMMA is the need to add radiopaque agents which lower its viscositywith unclear consequences on its long-term endurance.

In both higher pressure cement injection (vertebroplasty) andballoon-tamped cementing procedures (kyphoplasty), the methods do notprovide for well controlled augmentation of vertebral body height. Thedirect injection of bone cement simply follows the path of leastresistance within the fractured bone. The expansion of a balloon alsoapplies compacting forces along lines of least resistance in thecollapsed cancellous bone. Thus, the reduction of a vertebralcompression fracture is not optimized or controlled in high pressureballoons as forces of balloon expansion occur in multiple directions.

In a kyphoplasty procedure, the physician often uses very high pressures(e.g., up to 200 or 300 psi) to inflate the balloon which first crushesand compacts cancellous bone. Expansion of the balloon under highpressures close to cortical bone can fracture the cortical bone, orcause regional damage to the cortical bone that can result in corticalbone necrosis. Such cortical bone damage is highly undesirable andresults in weakened cortical endplates.

Kyphoplasty also does not provide a distraction mechanism capable of100% vertebral height restoration. Further, the kyphoplasty balloonsunder very high pressure typically apply forces to vertebral endplateswithin a central region of the cortical bone that may be weak, ratherthan distributing forces over the endplate.

There is a general need to provide systems and methods for use intreatment of vertebral compression fractures that provide a greaterdegree of control over introduction of bone support material, and thatprovide better outcomes. Embodiments of the present invention meet oneor more of the above needs, or other needs, and provide several otheradvantages in a novel and non-obvious manner.

SUMMARY OF THE INVENTION

The invention provides a method of correcting numerous boneabnormalities including vertebral compression fractures, bone tumors andcysts, avascular necrosis of the femoral head and the like. Theabnormality may be corrected by first accessing and boring into thedamaged tissue or bone (FIG. 1). Thereafter, the implant body isadvanced into the bore, wherein successive axial segments of the implantengage one another to provide radial expansion of the implant. Theimplant system eliminates to need to ream, compact, tamp or otherwisetreat the bone region before the insertion of the implant assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed description, similar reference numerals areused to depict like elements in the various figures.

FIG. 1 is a side view of a segment of a spine with a vertebralcompression fracture that can be treated with the present invention,showing an introducer in posterior access path to the targeted treatmentsite.

FIG. 2A is a greatly enlarged view of the reticulated material of theexpandable implants corresponding to the invention.

FIG. 2B is a side view of a segmented reticulated implant bodycorresponding to the invention.

FIG. 3A is a sectional perspective view of one of the reticulatedimplant segments of FIG. 2B taken along line 3-3 of FIG. 2B.

FIG. 3B is a view of a helical element that is a component of thereticulated implant segment as in FIG. 3A.

FIGS. 4A-4C are schematic views of a method of the invention in treatinga fracture as depicted in FIG. 1.

FIG. 5A is a cross-sectional view of the vertebra and fracture of FIG. 1with multiple implants deployed in a targeted region from a singleaccess.

FIG. 5B is a cross-sectional view of the vertebra and fracture of FIG. 1with multiple implants deployed from two access paths.

FIG. 6 is a schematic view of the damaged vertebra of FIG. 1 with thereticulated implant monolith I in place after expanding the height ofthe damaged vertebrae and providing a reticulated scaffold therein.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises a bone implant of a reticulated materialthat can be radially expanded in cross-section to support a bone, andmore particularly to move apart cortical endplates to at leat partlyrestore vertebral height. In one embodiment, the implants elementscomprise reticulated scaffold structures that can allow for later boneingrowth. FIG. 2A illustrates a greatly enlarged view of an exemplaryopen cell reticulated material 10 corresponding to the invention that isuseful for defining terms used herein to describe the implants. Ingeneral, the term reticulated means having the appearance of, orfunctioning as wire network or substantially rigid net structure. Therelated term reticulate mean resembling or forming a network. The termsreticulated, reticulate and trabecular are used interchangeably hereinto describe structures having ligaments 15 that bound an open cells 18or closed cell in the interior structure.

Such reticulated structures 10 as in FIG. 2A are further defined bydensity—which describes the ligament volume as a percentage of a solid.In other words, the density defines the volume of material relative tothe volume of open cells in a monolith of the base material. As densityof the ligaments increase with larger cross-sections and smaller celldimensions, the elastic modulus of the material will increase.

The cells of reticulated structure 10 (FIG. 2A) also define a mean crosssection which can be expressed in microns. In preferred embodiments, thecells are bounded by polyhedral faces, typically pentagonal orhexagonal, that are formed with five or six ligaments 15. Cell dimensionis selected for enhancing tissue ingrowth, and mean cell cross-sectionscan range between 10 micron and 300 microns; and more preferably rangesbetween 20 microns and 200 microns.

In general, the system and method of invention relates to minimallyinvasive percutaneous interventions for providing bone support with animplant scaffold that is expanded in situ into a rigid, reticulatedmonolithic body. FIG. 1 illustrates a spine segment 100 with vertebrae102 a and 102 b and disc 104. The vertebra 102 a and cancellous bone 106therein is depicted as having suffered a compression fracture 108 thatcan be repaired with the present invention. The targeted treatmentregion 110 (FIG. 1) can be accessed in either posterior or anteriorapproach A with a minimally invasive access pathway, for example with anintroducer 115 having a 5 mm. or smaller diameter. In other embodiments,the introducer and implants are preferably less than about 3 mm. incross-section.

Now referring to FIG. 2B, an exemplary radially expandable implant body120 is illustrated in a first pre-deployed configuration. In thisconfiguration, it can be seen that the elongate body 120 comprises aplurality of axial-extending segments 122 a-122 d that extend along axis125. In a preferred embodiment, the segments 122 a-122 d are linked bybreak-away joints. Alternatively, the segments are cannulated andcarried about a shaft indicated at 128. As another alternative, thesegments 122 a-122 d are independent bodies that are introducedsequentially through introducer sleeve 130. The segments 122 a-122 d areconfigured for axial segment-to-segment insertion to thereby radiallyexpand the assembled implant body. The expandable implant can includesfrom 2 to 20 body segments.

As illustrated in FIG. 2B, the radially expandable implant body 120 ishoused in an introducer 130 that comprises a sleeve extending from ahandle (not shown). The introducer and handle carry a rotational/helicaladvancement mechanism for engaging the proximal end 132 of the implant,and preferably includes a motor-driven mechanism.

FIGS. 3A and 3B are views of a segment 122 a of the radially expandableimplant body 120. In FIG. 3A, it can be seen that each segment consistsof a plurality of segment subcomponents or elements 140 (collectively)that are configured to helically intertwine.

FIG. 3B illustrates a single segment element 140 de-mated from thecooperating elements. The element is of a metal or polymer that issomewhat deformable. The elements can further be configured withweakened planes, relaxing cuts, hinge or flex portions 144 or the liketo allow bending of the elements to permit controlled deformation andexpansion of the body. In preferred embodiments, the expandable implant120 has segments with a surface spiral thread. In another preferredembodiment, the hinge or flex portions 144 comprise flexible reticulated(open cell) polymer that couples together the rigid reticulated metalportions. Thus, the scope of the invention includes for the first timeinventive means for fabricating a deformable but non-collapsiblereticulated implant body.

The elements 140 are fabricated of a reticulated material 100 (FIG. 3A)that has a ligament density ranging between 20 percent and 90 percent.More preferably, the reticulated material has a ligament density rangingbetween 20 percent and 50 percent. In one embodiment, the reticulatedmaterial includes titanium, tantalum, stainless steel or a magnesiumalloy that is biodegradable. In another embodiment, the reticulatedmaterial is at least partly a polymer, which can be at least one ofbioerodible, bioabsorbable or bioexcretable.

FIGS. 4A-4C illustrate an exemplary sequence of utilizing a 3 or 4segment implant to create a radially expanded reticulated monolith 120′.For example, a pre-deployed implant as in FIG. 2B can have a diameter ofabout 4 mm—and can be expanded to a selected oval or roughly sphericalshape having a diameter of 12 to 16 mm. The expandable, reticulatedsegments can have common right-hand or left-hand surface spiral threadconfigurations. Alternatively, the segments can have differentright-hand and left-hand surface spiral thread configurations.

It can be easily understood that an expandable implant can have variableopen cell cross sections, or a radial gradient in open cell crosssections by providing different segment with different densities andcell dimension.

An expandable implant also can have reticulated material provided withopen cells filled with an infill polymer. Such an infill polymer is atleast one of bioerodible, bioabsorbable or bioexcretable.

The method of the invention include treating, or prophylacticallypreventing, a fracture in a bone structure and comprised the steps ofintroducing a reticulated body of a deformable reticulated elements in afirst cross-sectional dimension into the bone structure. Thereafter, theimplant body is expanding into a reticulated monolith to a secondincreased cross-sectional dimension as depicted in FIG. 6. The methodincludes expanding the implant to replace or displace trabecular bonewith the reticulated body. The method includes expanding the implant tosupport cortical bone adjacent the reticulated body, and to jack apartthe cortical vertebral endplates.

Of particular interest, the expandable implant may be used in aprophylactic manner with small introducers, for example, to provide bonesupport in vertebrae of patients in advance of compression fracture.FIGS. 5A and 5B indicate that the treatment site 110 can be a singleregion accessed from a single access pathway or multiple spaced apartregions, for example accessed by two minimally invasive access pathways.

The expandable implant of the invention also can be used as a fusionimplant to controllably expand in an intervertebral space tocontrollably engage first and second vertebral bodies. The system allowsfor less invasive access to a targeted site for such a fusion implant.

Reticulated metals are available from ERG Materials and Aerospace Corp.,900 Stanford Avenue, Oakland Calif. 94608 or Porvair Advanced Materials,Inc., 700 Shepherd Street, Hendersonville N.C. 28792.

In any embodiment, the implant segments 122 a-112 d further can carry aradiopaque composition if the material of the implant itself is notradiovisible.

In any embodiment, the segments further can carry any pharmacologicalagent or any of the following: antibiotics, cortical bone material,synthetic cortical replacement material, demineralized bone material,autograft and allograft materials. The implant body also can includedrugs and agents for including bone growth, such as bone morphogenicprotein (BMP). The implants can carry the pharmacological agents forimmediate or timed release.

The above description of the invention intended to be illustrative andnot exhaustive. A number of variations and alternatives will be apparentto one having ordinary skills in the art. Such alternatives andvariations are intended to be included within the scope of the claims.Particular features that are presented in dependent claims can becombined and fall within the scope of the invention. The invention alsoencompasses embodiments as if dependent claims were alternativelywritten in a multiple dependent claim format with reference to otherindependent claims.

1. An expandable implant comprising an elongated body extending along anaxis, the body including a plurality of deformable helically-matedelements.
 2. An expandable implant as in claim 1 wherein thehelically-mated elements are at least partly of a metal.
 3. Anexpandable implant as in claim 2 wherein the metal includes at least oneof titanium, tantalum or a magnesium alloy.
 4. An expandable implant asin claim 1 wherein the helically-mated elements are at least partly of apolymer.
 5. An expandable implant as in claim 4 wherein the polymer isat least one of biodegradable, bioabsorbable or bioexcretable.
 6. Anexpandable implant as in claim 1 wherein the body comprises a pluralityof body segments configured for axial segment-to-segment insertion tothereby radially expand the body.
 7. An expandable implant as in claim 6wherein the body includes from 2 to 10 body segments.
 8. An expandableimplant as in claim 6 wherein the body segments are configured with aspiral thread surface.
 9. An expandable implant as in claim 8 whereinthe body segments have a common right-hand or left-hand spiral threadsurface.
 10. An expandable implant as in claim 8 wherein body segmentshave different right-hand and left-hand surface spiral thread surfaces.11. An expandable implant as in claim 1 wherein the body has open cells.12. A method of treating or prophylactically preventing a fracture of abone comprising the steps of (a) helically introducing into bone a firstbody including a plurality of deformable helically-mated elements, and(b) helically introducing at least one additional body that include aplurality of deformable helically-mated elements into the interior ofthe first body thereby deformably radially expanding the assembly tosupport the bone.
 13. The method of claim 12 wherein steps (a) and (b)displace cancellous bone.
 14. The method of claim 14 wherein steps (a)and (b) move cortical bone.
 15. The method of claim 12 wherein the boneis a vertebra.
 16. The method of claim 14 wherein steps (a) and (b) atleast partially restores vertebral height.
 17. A method for treating anabnormality in a bone comprising (a) introducing an implant body into orproximate a bone wherein the implant body includes a biodegradablemagnesium alloy, and (b) allowing the magnesium alloy to biodegradethereby creating space for tissue ingrowth.
 18. The method of claim 17wherein the implant body is deformable and further including the step ofradially expanding the deformable implant body.
 19. The method of claim17 wherein the implant body is configured for implantation in avertebra.
 20. The method of claim 17 wherein the implant body isconfigured for implantation in an intervertebral space.